>Today, though - which may be unrelated - it prompted me to check the
>certificate, which weirdly seemed to belong to my VPS provider; it
>wasn't the one configured in dovecot.
>Has anyone else seen either of these issues? My VPS provider hasn't
>up with any ideas yet.
They have a very clear idea - they are snooping on your TLS, and that doesn't
happen by accident.
It might be a sad accident with a firewall, but they should fix that in no
time. If not, you are best off finding a new provider.